The field of the invention is that of pulse modulation. More particularly, the invention relates to the control of the frequency and width of pulses for the control of switches, and particularly for power switches. Its field of use is in particular that of control systems with a variable structure (control by hysteresis, direct control of coupling (in the DTC angle: xe2x80x9cDirect Torque Controlxe2x80x9d)) and modulations with width and/or frequency of pulses (MLI).
The modern electrical devices are comprised principally by four modules shown in FIG. 1.
The source 11 can be continuous, monophase alternating or triphase. The power converter 12 ensures the function of adaptation of the source to the supply type necessary for the good operation of the load 13:
continuous constant source converted into a monophase alternating source with variable frequency and amplitude;
triphase alternating source converted into a continuous source of variable amplitude;
etc.
The management of this conversion of type of source is ensured by the control module 14. This module integrates in general two functions:
subjection (or regulation) of one or several sizes of load,
conversion of outputs of the regulators into control signals of the power switches.
The control systems with variable structure are more and more used, relative to conventional controls. This technique is also known as control by sliding mode or direct couple control DTC.
There can be distinguished two general types of control systems with variable structure, respectively in FIGS. 2 and 3.
FIG. 2 shows the case of a structural change by switching of a counter-reaction of condition. The electrical system 21 receives the voltage from an amplifier 22 controlled by a reference voltage delivered by one or another of the modules K1 and K2 (231 and 232), as a function of the switch 24 controlled by the commutation law S(x)25.
FIG. 3 shows the case of a change of structure by switching to the level of the amplifier (power converter). The commutation law S(x) then controls the switch 31, which delivers the selected voltage Emax or Emin.
The commutation law S(x) is derived from a relationship between the reference and the variables of the condition of the system, so as to ensure the stability, the durability and the observance of the reference, namely:
u=EmaxsiS(x) greater than 0
u=EminsiS(x) less than 0
This control has the advantage of a very rapid dynamic and a very good strength as to parametric variations. It is possible that the switching takes place at a very high frequency (theoretically infinitely high) such that the system works in the sliding mode.
However, this aspect turns out to be a major drawback in practice. Thus, there is no control over the frequency which requires this type of control for the switches of the power converter 12 which supplies the electrical supply. These frequencies can in certain cases be damaging to the static converter.
This problem is well known. Numerous methods seeking to solve it have already been proposed:
replacing the outlet relays of the regulator with hysteresis, with a band calculated as a function of the parameters of the system, associated with control systems of the width of the hysteresis band as a function of the operating point;
adding an auxiliary estimator and a regulator of the switching frequency. The parameters of this xe2x80x9cfrequency controllerxe2x80x9d are computed as a function of the parameters of the system to be controlled;
breaking down the regulator xe2x80x9cwith variable structurexe2x80x9d into two sub-regulators: one is dedicated to the linearization of the model of the system to be controlled and the second is an image of the base regulator. It imposes the dynamic in closed loop and the durability for small variations of parameters.
Moreover, in the case of the regulation of the currents of the triphase motor with alternating current, there exist two typical solutions, control by MLI (modulation of the width of the pulses) and control by hysteresis.
The source is the continuous voltage type (which is to say a mean value which is not zero, in contrast to alternating voltage), the load is a synchronous or asynchronous triphase motor and the power converter is an inverter of triphase voltage (conversion from a constant continuous voltage into three alternate voltages of variable frequency and amplitude). The control means must be subject to the current from the motor with three alternative references, for example sinusoidal.
The principle of the control by MLI is given in FIG. 4.
The error between the reference current 41 and the measured current 42 is processed by a corrector 43. The control of the power switches 44 and 45 is obtained by the comparison 46 of the output of the regulator 43 and the triangular signal 47 of very high frequency relative to that of the reference currents (40 to 100 times, or even more).
This type of control ensures switching of the power switches at a constant frequency (frequency of the triangular signal of modulation), but the inversion of the current is not controlled, depending on the parameters of the load and of the point of operation. The synthesis of the regulator is in general based on the linear automatic control, which introduces an inherent dephasing into the linear transfer functions, at least of using a sophisticated corrector requiring a fairly powerful processor or a very complicated analog card.
In all cases, the quality of direct control is strictly connected to the fineness with which the parameters of the model of the system have been predetermined.
A second method utilized to render more sensitive, relative to the frequency of the current, the performances of the direct control, consists in carrying out a base change by means of a non-linear transformation matrix (so-called Park transformation), which transforms the alternative sizes into continuous components (constants) in the new base. The currents being continuous in this base, the corrections are determined to ensure good performance at zero frequency.
This method permits avoiding the problem of variable frequency of the references for the production of current regulators, but the sensitivity to the parameters of the model of the system remains unchanged relative to the previous method.
The control principle by hysteresis is shown in FIG. 5. It consists in holding by means of three hysteresis comparators 51 (in the case of a triphase system) the real currents in the machine within a band of predetermined width, centered on the reference currents.
The inversion of the current is thus imposed but the frequency of switching is free and variable. It depends principally on the bandwidths imposed and the time constants of the system to be controlled (the motor in this case). This method has the following advantages:
a simple hysteresis comparator 51 permits directly controlling the currents and generating the controls for the switches of the power converter;
the direct control is not linear, which leaves the possibility of making almost zero the dephasing and minimizing the gain error between the reference currents and the real currents;
the performances of the direct control are less sensitive to the parameters of the model of the system.
Furthermore, they have several drawbacks, connected particularly to the poor control of the frequency of switching of the power switches piloted by the control signals 44 and 45;
switching constraints are very important at the level of the power converter (heating, defects of switching, . . . );
the variation in frequency of switching can be a source of troublesome audible noises.
None of these known techniques is therefore satisfactory. Thus, they all depend on the parameters of the system to be controlled, which of course degrades the advantages reached by the control with variable structure, particularly in terms of universality.
Moreover, they are often costly. They require the use of complex electrical means, even of very rapid processors.
The invention particularly has for its object to overcome these drawbacks of the prior art.
More particularly, the invention has for its object to provide a control technique for switching of static converters in a control systems with a variable structure which has very little sensitive to variations of the parameters of the system to be controlled, even practically independent of these latter, as a first approximation.
Another object of the invention is to provide such a technique, which will be simple and less costly to use.
The invention also has for its object supplying such a technique, which can be implanted in an integrated circuit.
The invention also has for its object to provide such a technique, which will be compatible with any type of system, and particularly which can be used as well in monophase systems and in polyphase systems.
These objects, as well as others which will become apparent from what follows, are achieved, according to the invention, with a control process of the durations and periods of conduction of at least one power switch in a control system with a variable structure, which system is of the type comprising at least one switch actuable to deliver a given electrical signal corresponding to an electrical system, as a function of a control system taking account of an input reference signal and feedback signal from an output of said electrical system, according to which process there is applied to said control signal an oscillation with controllable frequency, obtained by use of a filter on said feedback signal, said filtering being defined so as to introduce a dephasing of xe2x88x92180xc2x0 between the feedback and filtered signal on the one hand, and the input signal of the electrical system on the other hand, at said controllable frequency.
In other words, according to the invention, there is produced a modulation of the control signal. It can thus be determined that the return channel ensures conjointly two functions: a subjection function, because the control signal will follow the input reference signal, and an oscillation function, which permits limiting to a maximum given frequency the swinging of the switches.
It is to be noted that this approach is altogether new for those skilled in the art and that, more precisely, it goes against the prejudices of these latter. Thus, specialists of direct controls always seek, in such systems, to avoid oscillations, considered as synonymous with instability.
The invention also relates to control devices for at least one power switch using such a process. Such a device therefore comprises means for applying to said control signal an oscillation at a controllable frequency obtained by the use of a filter on said feedback signal, said filter being defined so as to induce a dephasing of xe2x88x92180xc2x0 between the feedback and filtered signal on the one hand, and the input signal of said electrical system on the other hand, at said controllable frequency.
This result is obtained with the help of a function disposed between the output and the input of amplification means. In a particular embodiment, it can be obtained with the help of an electrical analog filter whose self-frequency is adjacent said controllable frequency.
According to embodiments and applications, said controllable frequency can be substantially constant, or adjustable. In this latter case, said filter means can for example comprise a filter with switched capacitors.
Moreover, the device according to the invention can be applied equally well to monophase systems as to polyphase systems.
In the case of a polyphase system, the device preferably comprises, for each of the phases:
a filter for the feedback signal, said filter being defined so as to induce a dephasing of xe2x88x92180xc2x0 between the feedback and filtered signal on the one hand, and the input signal of the electrical system on the other hand, at said controllable frequency, and
a combined low pass and high pass filter, the low pass filter supplying a first comparator and the high pass filter supplying a second comparator, delivering said control signal.
The device of the invention can be provided with the help of analog means, which permit obtaining a very rapid and relatively inexpensive assembly.
It can also, particularly in the case of polyphase systems, be provided with analog and digital processing means. Again, there can also be obtained better performance than with known completely digital systems.
Preferably, the device of the invention is implanted in an integrated circuit. This is possible, because of the relative simplicity of use of the invention. There can thus be produced a single component, finding use in numerous fields.
It will thus be noted that, contrary to the main techniques, the process, the device and the system of the invention do not require a precise recognition of the parameters of the load to be supplied. In other words, the approach of the invention requires at most, as a first approximation, a knowledge of the order of magnitude of the system to be controlled.
The invention also concerns of course the systems for controlling with variable structure, using such a device.